In recent years, exhaust controls on carbon dioxide gas and other substances have become stricter as actions to safeguard the environment are increased. In the motor vehicle industry, therefore, the development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has become accelerated as a substitute for vehicles using fossil fuel such as gasoline, diesel oil, and natural gas. Nickel-hydrogen secondary batteries and lithium-ion secondary batteries have been used as batteries for EVs and HEVs. In recent years, nonaqueous electrolyte secondary batteries such as lithium-ion secondary batteries have been used more often because of their light weight and high capacity. For such a nonaqueous electrolyte secondary battery, an outer body of aluminum-laminated film is proposed because it enables an easy increase in size and decrease of the cost of material.
It is required for the batteries for EVs and HEVs to respond to the improvement of basic performance for automobiles, namely, driving performance such as accelerating performance and hill-climbing performance, as well as environmental friendliness. Furthermore, it is required to prevent degradation of the driving performance even in severe environments (usage in very cold areas and very hot areas).
It has been proposed to add vinylene carbonate and difluorophosphate to a nonaqueous electrolyte in order to improve low-temperature discharge characteristics of the nonaqueous electrolyte secondary battery (refer to JP-A-2007-141830).
However, batteries for EVs and HEVs are used in various kinds of environments, which require further improvement.